1. Field of the Invention
The present invention relates to a color system discriminator for automatically discriminating a color system from a tuner detection signal, and more particularly to a color system discriminator that has improved accuracy of discrimination.
2. Description of the Related Art
Color system having various specifications such as the NTSC (National Television System Committee) scheme, the PAL (Phase alternation by Line) scheme, and the SECAM (Sequential a memoir) scheme is employed in various places all over the world. Accordingly, if the color system of an input tuner detection signal is different from that of a color demodulating circuit provided with the television, a television image would be out of phase or provided with no color. Thus, in order to make the television available for worldwide use, it is therefore necessary to equip the television with a color demodulating circuit for each type of worldwide color system to make the television compatible with the various types of the color system. It is also necessary to select an available color demodulating circuit by automatically discriminating a color system indicated in an input tuner detection signal or by setting the color system manually.
However, to set the color system manually, the user needs to have knowledge specific to the operation of the television. Furthermore, suppose that the television is used where several types of color system are employed and the color system indicated in a tuner detection signal to be inputted into the television is changed from time to time. In this case, it is necessary to set the color system manually each time it changes. These operations are extremely annoying for the user. Accordingly, the television system equipped with a color system discriminator for automatically discriminating between types of the color system has been desired.
A prior method for automatically discriminating a color system in the television is as follows. For example, a color system is determined based on information such as the burst lock frequency of the color system from the input tuner detection signal or the phase of the demodulated R-Y color difference signal.
However, in some cases, an input tuner detection signal that is weak in level or received in a bad condition due to noise would cause the burst lock frequency to be detected in an erroneous manner. In such a case, it could happen to select a color demodulating circuit that is different from the one based on the color system indicated in the input tuner detection signal. In particular, an input tuner detection signal that is weak in level would cause the television to detect the correct burst lock frequency only intermittently in some cases. In such a case, the setting of the color demodulating circuit of the television is changed intermittently. This would cause intermittent color flashing to appear on the screen and the vertical synchronization to become unstable leading to an out-of-phase image appearing on the screen.
It is an object of the present invention to provide a color system discriminator that can reduce erroneous detection of the color system indicated in an input tuner detection signal.
According to the present invention, a color system discriminator comprises: a frequency detector which detects whether a frequency of a sound carrier signal included in a tuner detection signal is 4.5 MHz; a vertical-synchronizing frequency detector which detects a vertical synchronizing frequency of an image signal included in the tuner detection signal; a plurality of color demodulators which demodulate the image signal in accordance with a predetermined type of the color system different from one another; a demodulating switch which outputs the tuner detection signal to any one of the plurality of color demodulators; and a controller which controls the demodulating switch. Each of the color demodulators determines whether the image signal is based on the predetermined type in accordance with a burst lock frequency of the image signal. The controller determines a type of the color system in accordance with a detection result provided by the frequency detector, the vertical synchronizing frequency, and a determination result provided by the color demodulators.
According to the present invention, the type of the color system is determined with reference not only to the burst lock frequency indicated in the tuner detection signal and the phase of the R-Y color difference signal of the image signal, but also to the frequency of the sound carrier signal and the vertical synchronizing frequency. This makes it possible to determine the color system in accordance with a plurality of conditions, thereby allowing the erroneous detection of types of the color system indicated in the tuner detection signal to be reduced.